1. Field of the Invention.
The present invention relates to corrosion inhibition of zinc bromide/calcium halide-based clear, high density fluids used in the completion and work over of oil and gas wells and more particularly to corrosion inhibitors incorporating a synergetic combination of thio group of corrosion inhibitors and an aldose group antioxidant.
2. Description of the Prior Art.
In oil and gas well drilling, a completion or workover fluid is any fluid that is placed across a production zone before or immediately after perforation, or a fluid placed across the formation during reworking operations such as underreaming, gravel packing, or any remedial work that must be accomplished in efforts to obtain increased production. Its primary functions are to provide pressure control, to prevent formation fluid from coming into the well bore, to maintain hole stability, to minimize formation of production damage, to minimize corrosion to downhole metals during the time of contact, and to provide carrying capacity for debris, cuttings, or loose sand that is encountered.
Because they are solids free and discourage the swelling of clays, brines of wide density range brines are utilized as completion or workover fluids. However, such brines are frequently more corrosive than other fluids. Substantial corrosion of metal drill pipe is often experienced when drilling into high temperature subterranean formations. The high temperature bottom hole condition combined with the corrosive brines and high dissolved oxygen levels, are conducive to excessive corrosion of drill pipe, casing, and other metal equipment. The life expectancy of drill pipe employed under these conditions is relatively short, requiring frequent inspection and replacement. Furthermore, an even more serious problem is the failure of drill pipe in service where it breaks leaving a lower section of the drill pipe in the well. This failure interrupts the drilling operation, often requiring expensive fishing operations. An unsuccessful fishing operation results in the pipe remaining in the well and the well being abandoned.
Deeper and higher temperature wells require higher density brines. Brines such as zinc halide can meet this requirement. However, corrosion problems are magnified with zinc halide completion or workover fluids. In sodium or calcium based fluids, corrosion can be controlled to an acceptable level by raising the pH, adding film forming amine, or admixing with oxygen scavengers. However, in the case of zinc based fluids, the pH cannot be raised without the undesirable side effect of precipitating the zinc ion out of solution. Film-forming amine based corrosion inhibitors are either not effective enough or not soluble enough, and conventional oxygen scavengers such as hydrazine and sulfites form precipitates or leave insoluble residues which will damage formation of production zones.
Although conventional corrosion inhibitors such as ammonium thioglycolate and ammonium thiocyanate reduce the corrosion of iron and steel casing, tubing and equipment exposed to the high temperature and concentrated aqueous solutions of zinc bromide, calcium bromide and calcium chloride, they have not solved the problem completely.
More particularly, Canadian Pat. No. 1,197,673 describes in detail a corrosion inhibiting system for zinc-based fluids. It describes several corrosion inhibitors which provide corrosion protection to iron and steel casing, tubing and equipment exposed to high temperatures, and highly concentrated aqueous solutions of zinc bromide, calcium bromide and calcium chloride used as completion, workover or packer fluids. The inhibitors are effective in aqueous zinc based fluids in the density range of 15.0 to 19.2 lb/gal at temperatures up to and including 400.degree. F. (204.degree. C.), and up to 90 days. Pitting corrosion is minimized when the inhibitors are added to 19.2 lb/gal zinc bromide/calcium bromide fluids. The preferred composition contains 1.0 weight percent ammonium thioglycolate (50-60% solution). Even though these inhibitors reduce corrosion, the problem has not been solved completely.
European Pat. No. 0,153,192 describes corrosion inhibitors for use in the protection of metals from the corrosive effect of heavy brines, particularly zinc halide brines, employed in the drilling, completion and workover of well bores penetrating subterranean formations. The monovalent and divalent salts of erythorbic acid, when admixed in a sufficient amount with a heavy brine solution, are disclosed to inhibit the corrosive effect of heavy brines on metals in contact with the solutions, and do not precipitate from the brines upon addition thereto, or upon a change in pH or temperature of a solution. The corrosion inhibiting effect is proposed by the inventors to take place through several mechanisms. Since erythorbic acid is used as an effective oxygen scavenger in boiler water systems, the inventors speculated that one of the inhibiting mechanisms is an oxygen scavenging process. None of the mechanisms described in the patent is related to the synergic scavenging effect of the erythorbic acid in the presence of thio containing inhibitors.
U.S. Pat. No. 3,634,270 describes a mixture of organic nitrogen and sulfur compounds which prevents the corrosive ingredient of the solution from attacking metal. Application to industrial boiler and heat exchanger cleaning is suggested. Sulfur compounds such as thiourea, allylthiourea, sodium mercaptobenzothiazol, mercaptothiazolene, sodium thiocyanate, and mixtures thereof are suggested to be particularly effective. Some of the nitrogen compounds which are to be particularly effective are normal alkylamine, having 1 to 20 or more carbon atoms in the alkyl substituent, iso-alkyl and ter-alkylamines having up to 20 carbon atoms in the alkyl substituent, di-alkylamines having 1 to 20 alkyl groups in the alkylsubstituents, diamines, ammonias, ureas, thioureas, amides, ammonium salts, and alicyclic, hetrocyclic, and aromatic amines. The inhibitor systems were tested at temperatures no greater than 300.degree. F. and for periods no longer than 16 hours. Such short term protection is of little use in oil well completion or workover fluids. In addition, most of the amines claimed to be effective are not sufficiently soluble in the highly concentrated zinc bromide and calcium bromide brines.
In addition, bisulfites, sulfites, and hydrazines are oxygen scavengers commonly used in the oil fields. While sodium and ammonium salts of sulfites and bisulfites show some scavenging effects and reduced corrosion rates somewhat, they also form insoluble residues in the fluids. Hydrazine is not effective in reducing oxygen concentration and also forms a white precipitation. Other scavengers such as methyl-ethyl ketoxime, cyclohexylamine, N-N-diethylhydroxylamine, carbohydrazide and morpholine, etc., are not soluble enough to be effective for oil field use.
Thus, a primary object of the present invention is to develop an effective corrosion inhibitor composition for use in zinc halide-based clear fluids.
A further object is to provide a composition of the character described, incorporating a synergistic thio group-corrosion inhibitor aldose group antioxidant system which will effectively control corrosion in zinc bromide/calcium halide fluids.